It is desirable from the points of view of reducing NO.sub.x emissions and improving engine fuel consumption to dilute the mixture supplied to the combustion chambers either by making the mixture lean (air dilution) or by recirculating exhaust gases (EGR dilution). The dilution slows down the burn rate and reduces the gas temperature at the end of combustion and this reduces NO.sub.x formation. Also, the dilution reduces the output power and the engine throttling must be reduced to maintain the same power, which results in reduced pumping losses and improved fuel economy at a given power output.
There is a limit to which the mixture can be diluted with air and/or EGR gases because beyond this limit hydrocarbon emissions become excessive and ultimately the engine becomes unstable and prone to misfire. Engines therefore require careful calibration of the dilution to reduce emissions and improve fuel economy without sacrificing combustion stability.
It is common practice to use both lean burn and EGR dilution in combination and this results in high complexity in the engine calibration because of the number of variables, all of which are interrelated.
In many prior art systems, calibration is achieved by first setting a desired AFR (air to fuel ratio) and subsequently adding EGR dilution to the point where instability commences. This however assumes that the degree of EGR dilution can be controlled rapidly and accurately, which even with the use of closed loop EGR metering systems is not necessarily the case.
The reason why closed loop EGR control is ineffective is that the pressure difference between the intake manifold and the exhaust system varies significantly and rapidly during normal engine operation. At light load, the intake manifold vacuum is high and only a small proportion of EGR dilution is permissible and therefore significant flow restriction is required in the EGR metering system. On the other hand, at higher loads, the manifold vacuum drops while the demand for EGR dilution increases. The net result is that an EGR metering system that is capable of maintaining good accuracy at light load is incapable of meeting the EGR demand at higher loads because of excessive restriction in the EGR metering system.
The control steps in a closed loop control system relying on sensors and intervening actuators also result in slow response so that when the main air flow changes rapidly during transients, the EGR dilution cannot follow at the same rate with the result that the dilution setting is disturbed during the transients.
All these problems make conventional EGR metering systems poor in accuracy and response, expensive and unreliable.